Dissolved organic matter in surface sediments along a river-to-ocean continuum: Molecular characteristics and...

Dissolved organic matter in estuarine sediments (SDOM) plays a crucial role in coastal carbon cycles, mediating the transformation, exchange, and sequestration of carbon between sediments and overlying waters. However, the mechanisms that govern the transformation and fate of SDOM in estuarine environments remain poorly understood. In this study, we conducted a molecular composition analysis of SDOM in the surface sediments of the Changjiang Estuary-East China Sea continuum, using ultrahigh-resolution mass spectrometry. Building on previous characterizations of stable and radioactive carbon isotopes in dissolved organic carbon and sedimentary organic carbon from surface sediments and bottom waters, this study aimed to track the provenance and age of SDOM at the molecular level. Additionally, by comparing with bottom-water DOM, we sought to elucidate the exchange dynamics of SDOM at the sediment-water interface. Our results indicate that SDOM is more biologically labile than bottom-water DOM, characterized by an enrichment in aliphatic, low-molecular-weight, nitrogen-containing compounds that enhance its susceptibility to microbial degradation. Frequent hydrodynamic disturbances and sediment resuspension in the dynamic mobile mud zone of the inner shelf create a hotspot for DOM exchange between sediments and overlying waters. Active vertical mixing facilitates the downward transport of fresh surface-derived DOM, while riverine particles repeatedly adsorb and release DOM, allowing for the concurrent sequestration and remobilization of both marine and terrestrial DOM. These findings highlight the essential role of SDOM as a highly reactive carbon pool that accelerates OM turnover in large river-dominated ocean margins. Furthermore, this research elucidates the rapid biogeochemical mechanisms driving carbon cycling in estuaries and reveals how SDOM either transforms or preserves OM in sediments, with significant implications for refining global coastal carbon budgets.